The present invention relates generally to optical inspection systems, and specifically to methods and systems for detecting defects on patterned substrates.
The use of spatial filtering is well known in systems for optical inspection of patterned substrates, which contain regularly-repeating structures. When the repetitive patterns on such substrates are illuminated with coherent light, they generate constructive interference lobes along well-defined directions. A suitable lens can be used to collect the light reflected from the surface into an image that constitutes a spatial Fourier transform of the substrate pattern. (The lens used for this purpose is referred to as a Fourier transform lens, and the plane in which the Fourier transform image is formed is referred to as the Fourier plane.) The position and extent of the interference lobes in the Fourier plane depend on the period of the pattern and on a scaling factor determined by the wavelength of the incident radiation and the focal length of the lens.
It is known in the art that blocking the interference lobes in the Fourier plane facilitates the detection of defects and pattern irregularities on the substrate. For example, U.S. Pat. No. 3,614,232, to Mathisen, whose disclosure is incorporated herein by reference, describes a spatial filter for detecting defects in photomasks, using a transmission geometry and a simple filter consisting substantially of the negative of the Fourier transform of a defect-free specimen of the microcircuit. U.S. Pat. No. 5,177,559, to Batchelder et al., whose disclosure is likewise incorporated herein by reference, describes a dark field imaging system for inspecting repetitively patterned integrated circuits on a semiconductor wafer. The light scattered from the pattern is filtered with an opaque spatial filter, which attenuates spatial frequency components corresponding to the wafer pattern, and is then converted to an image on an imaging sensor. The substrate is illuminated at a grazing angle to the substrate plane, whereas the scattered light is collected in a direction essentially normal to the surface.
U.S. Pat. No. 4,370,024, to Task et al., whose disclosure is incorporated herein by reference, describes a dynamic binary Fourier filtered imaging system (not specifically for wafer inspection.) The spatial filter used in this case consists of a liquid crystal array, which can be programmed to dynamically produce different opaque filter patterns. Further in this vein, U.S. Pat. No. 5,276,498, to Galbraith et al., whose disclosure is also incorporated herein by reference, describes a system for performing dark field surface inspection of substrates, such as repetitively patterned semiconductor wafers, employing a scanned, focused laser beam and an adaptive spatial filter consisting of a liquid crystal light valve array. The laser beam is incident at a grazing angle on the wafer surface, and the scattered light is also collected at a grazing angle (away from the specularly reflected optical axis), and is measured by a detector. The proper configuration of the spatial filter is determined by illuminating the repetitive pattern with the laser, successively turning each liquid crystal element on and off, and measuring the level of the optical signal at the detector. The pattern is then stored in a computer memory for subsequent use.
U.S. Pat. No. 5,659,390, to Danko, whose disclosure is incorporated herein by reference, describes a system for performing dark field surface inspection of substrates, such as repetitively patterned semiconductor wafers, employing a scanned laser beam and an adaptive spatial filter. Here the spatial filter consists of an optically-addressable liquid crystal spatial light modulator. The laser beam is incident at a grazing angle to the wafer surface, and the scattered light is collected by a detector in a direction normal to the surface. A write beam derived directly from the Fourier-transformed scattered light automatically determines the proper configuration of the spatial filter.
Solutions based on adaptive spatial filters, such as those described in U.S. Pat. Nos. 5,276,498 and 5,659,390, are advantageous in their ability to match the spatial filter to different substrate patterns without the need to replace an optical element. Such filters are capable of generating arbitrary filter patterns, within the constraints of liquid crystal technology. This technology suffers from several shortcomings, however:
1) The transmission range of liquid crystals known in the art is limited to wavelengths in the visible spectrum, roughly 400-700 nm.
2) Liquid crystal modulators have limited contrast ratios, and they are sensitive to polarization and to incidence angle. Therefore, the optical signal transmission of the desired light may be reduced, while the transmission of the undesired light is increased, hence reducing the contrast of the spatial filter itself.
3) When the optically-addressed mechanism is used (as in U.S. Pat. No. 5,659,390), the spatial filter is determined completely automatically. There is no possibility of user intervention to adjust the filter configuration to compensate for unusual substrate features and for different measurement conditions.
It is an object of some aspects of the present invention to provide improved methods and devices for spatial filtering, and particularly for adaptively blocking undesired spatial frequency components in the Fourier plane.
It is a further object of some aspects of the present invention to provide improved methods and systems for inspection of patterned substrates.
In some preferred embodiments of the present invention, a sample is scanned by a beam of coherent light, and light scattered from the substrate is collected by a Fourier lens. An image sensor acquires an image of the Fourier plane and inputs the image to a filter controller, typically a computer, which analyzes the image to determine an optimal spatial filtering pattern. The computer accordingly controls a programmable spatial filter in the Fourier plane to filter the scattered light using the desired filtering pattern. The use of the image sensor in this manner allows the filtering pattern to be calculated adaptively for the particular substrate and inspection conditions, while simplifying the determination of the filtering pattern, relative to methods known in the art, since the spatial filter itself is not involved in the determination. The filtered light is received by a detector, and the detector signal is analyzed to detect defects in the substrate.
Typically, the sample comprises a patterned substrate, and the optimal filtering pattern is determined, based on the acquired image of the Fourier plane, so as to block spatial frequencies that correspond to the interference lobes generated by the substrate pattern. Additionally or alternatively, the computer maintains a database of optimal filter configurations that have been determined empirically and/or theoretically for different substrate types and measurement conditions. Information from the database is preferably combined with the acquired image to determine the optimal filter pattern to use in each case, either automatically or under the control of an operator. Alternatively, when appropriate, the filter pattern stored in the database may be used, while the acquired image is disregarded. In any case, the filter pattern is preferably determined not only by the acquired image, but also by other considerations, which may be addressed in the database and/or controlled by the user.
In some preferred embodiments of the present invention, the spatial filter comprises an array of micro-optical-electro-mechanical (MOEM) elements. The computer controls the xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d status of each pixel in the filter by varying the tilt angle of the particular micro-element. In some of these embodiments, the micro-elements comprise mirrors, and the filter operates in a reflective mode. Only the light that is reflected from the xe2x80x9conxe2x80x9d pixels reaches the detector, while the light reflected from the xe2x80x9coffxe2x80x9d pixels is dumped. In other embodiments, the micro-elements comprise shutters, so that the filter operates in a transmissive mode. By comparison with transmissive spatial filters known in the art, however, such as liquid crystal arrays, the present MOEM filters have a much wider spectral range, including ultraviolet (UV) and infrared (IR) wavelengths, and are relatively insensitive to polarization and incidence angle.
Preferably, to reduce diffraction effects in embodiments using reflective spatial filters, the micromirrors are oriented so that in the xe2x80x9coffxe2x80x9d position they are oriented either parallel to the face of the array, or at a blaze angle chosen so that the diffracted light is concentrated in a lobe away from the detector. Further preferably, the direction of tilt of the micromirrors is perpendicular to the direction of scanning the coherent beam over the substrate, so that the direction of the reflected beam does not change during the scan.
Thus, preferred embodiments of the present invention provide methods of adaptive spatial filtering and substrate inspection that are more flexible and versatile than methods known in the art, in terms of both optical properties of the inspection system and optimization of the spatial filtering patterns that are used. These methods are particularly suited to detection of defects in patterned substrates, such as semiconductor wafers, but they are also useful in other inspection tasks. For example, for detecting particle-type defects, which are elevated above the plane of a substrate, it may be desirable to collect light only at grazing angles, while for defects of the pattern itself, such as missing features, collection at higher elevation angles may be desirable. To deal with these different inspection needs, preferred embodiments of the present invention permit the optical configuration and operating wavelengths of the inspection system to be varied freely, and allow the optimal filter configuration to be determined not only by the repetitive substrate pattern itself, but also by other considerations which may be addressed in the aforementioned database.
There is therefore provided, in accordance with a preferred embodiment of the present invention apparatus for optical inspection of a sample, including:
a radiation source, adapted to irradiate a spot on the sample with coherent radiation;
a Fourier lens, adapted to collect the radiation that is scattered from the spot and to separate the collected radiation into spatial components in a Fourier plane of the lens;
a programmable spatial filter, positioned at the Fourier plane so as to filter the spatial components of the radiation;
a detector, positioned to receive the filtered radiation from the spatial filter, and to generate, responsive to the radiation, a signal for processing to detect a characteristic of the sample;
an image sensor, optically coupled to capture an image of the spatial components of the collected radiation in the Fourier plane, while the components are incident on the filter; and
a filter controller, coupled to receive and analyze the image captured by the image sensor and, responsive thereto, to control the spatial filter so as to determine the spatial components to be received by the detector.
Preferably, the spatial filter includes an array of filter elements, which are individually controllable by the filter controller.
In a preferred embodiment, the filter elements include mirrors, which are controllable by the filter controller so as to tilt between a first orientation, in which the mirror reflects the radiation incident thereon toward the detector, and a second orientation, in which the mirror reflects the radiation incident thereon away from the detector. Typically, the array of the mirrors defines an array plane, and in the first orientation, the mirrors are tilted out of the plane, while in the second orientation, the mirrors are oriented substantially parallel to the plane. Alternatively, in the second orientation, the mirrors are oriented relative to the plane at a blaze angle chosen so as to maximize diffraction of the radiation from the mirrors into an order directed away from the detector.
Typically, the array of the mirrors has a periodic structure, and the detector preferably includes a radiation sensor and spatial filtering optics, which are configured to block the radiation diffracted from the array toward the sensor due to the periodic structure of the array, while focusing the radiation reflected from the mirrors in the first orientation onto the sensor.
In another preferred embodiment, the filter elements include shutters, which are controllable by the filter controller so as to move between a first orientation, in which the shutter allows the radiation to pass therethrough toward the detector, and a second orientation, in which the shutter blocks the radiation incident thereon from reaching the detector. Typically, each of the shutters has a respective clear aperture, and the spatial filter further includes an array of microlenses, respectively aligned with the shutters so as to focus the radiation through the clear aperture.
In a further preferred embodiment, the image sensor includes a matrix of sensor elements, and the apparatus includes imaging optics arranged to direct a portion of the radiation collected by the Fourier lens onto the image sensor so that the sensor elements are optically substantially registered with the filter elements, and the controller is adapted to control each of the filter elements responsive to the radiation collected by a respective one of the sensor elements with which the filter element is registered.
Preferably, the apparatus includes a beamsplitter, which is positioned to direct a portion of the radiation collected by the Fourier lens toward the image sensor so that the image sensor can capture the image of the spatial components.
Further preferably, the filter controller is adapted to determine respective intensities of the spatial components in the captured image, and to control the spatial filter responsive to the intensities. Most preferably, the filter controller is adapted to control the spatial filter so as to inhibit the spatial components whose respective intensities exceed a given threshold from reaching the detector.
In a preferred embodiment, the sample includes a substrate having a surface with a pattern formed thereon, so that the spatial components in the Fourier plane include one or more interference lobes formed in the scattered radiation due to the pattern, and wherein the filter controller is adapted to control the spatial filter so as to block at least one of the interference lobes. Typically, responsive to the at least one of the interference lobes being blocked by the spatial filter, the detector is able to detect a defect on the surface of the substrate.
Preferably, the apparatus includes a memory, which is configured to store filter configuration data with respect to different testing parameters, and the filter controller is adapted to receive the testing parameters relevant to the sample under inspection, to read the filter configuration data from the memory corresponding to the relevant testing parameters, and to control the spatial filter responsive to the configuration data. Typically, the sample includes a substrate of a given type, which has known spatial components in the Fourier plane, and the testing parameters include an identification of the substrate type, causing the filter controller to control the spatial filter responsive to the known spatial components, so as to facilitate identification of a defect in the sample.
There is also provided, in accordance with a preferred embodiment of the present invention, apparatus for spatial filtering, including:
a Fourier lens, adapted to collect radiation emitted from a point and to separate the collected radiation into spatial components in a Fourier plane of the lens;
a programmable spatial filter, positioned at the Fourier plane;
an image sensor, optically coupled to capture an image of the spatial components of the collected radiation in the Fourier plane, while the components are incident on the filter; and
a filter controller, coupled to receive and analyze the image captured by the image sensor and, responsive thereto, to control the spatial filter so as to block one or more of the spatial components.
Preferably, the spatial filter includes an array of filter elements, which are individually controllable by the filter controller, so that each filter element assumes a blocking or a non-blocking status.
There is additionally provided, in accordance with a preferred embodiment of the present invention, optical inspection apparatus, including:
a radiation source, adapted to irradiate a spot on a sample with coherent radiation and to scan the spot along a line on the sample in a scan direction;
a Fourier lens, adapted to collect the radiation that is scattered from the spot and to direct the collected radiation to a Fourier plane of the lens;
a programmable spatial filter positioned at the Fourier plane, the spatial filter including an array of mirrors that are controllable so that each mirror can be individually tilted about an axis parallel to the scan direction between a first orientation and a second orientation; and
a detector, positioned to receive the collected radiation reflected from the mirrors in the array that are in the first orientation, and not the radiation reflected from the mirrors that are in the second orientation, and to generate, responsive to the radiation, a signal for processing to determine a characteristic of the sample.
Typically, the array of mirrors has a periodic structure, and the detector includes a radiation sensor and spatial filtering optics, which are configured to block the radiation diffracted from the array toward the sensor due to the periodic structure of the array, while focusing the radiation reflected from the mirrors in the first orientation onto the sensor. Preferably, the spatial filtering optics include a second Fourier lens, which is arranged to collect the radiation diffracted from the array and reflected from the mirrors in the first orientation and to direct the collected diffracted and reflected radiation to a second Fourier plane, and an opaque target having a transparent opening, positioned at the second Fourier plane so as to block one or more orders of the diffracted radiation while passing the radiation reflected from the mirrors. Most preferably, the opening includes a slit oriented approximately parallel to the scan direction.
There is further provided, in accordance with a preferred embodiment of the present invention, a spatial filter, including an array of mirrors arranged in a plane, the array being controllable so that each mirror can be individually tilted between a first orientation, in which the mirror is tilted out of the plane so as to deflect radiation that is incident on the mirror toward a detector, and a second orientation, in which the mirror is oriented substantially parallel to the plane so as to deflect the radiation away from the detector.
There is moreover provided, in accordance with a preferred embodiment of the present invention a method for optical inspection of a sample, including:
irradiating a spot on the sample with coherent radiation;
collecting a portion of the radiation that is scattered from the spot, and separating the collected radiation into spatial components in a Fourier plane;
positioning a programmable spatial filter in the Fourier plane so as to filter the spatial components of the radiation;
receiving the filtered radiation from the spatial filter at a detector, and generating, responsive to the filtered radiation, a signal for processing in order to detect a characteristic of the sample;
capturing an image of the spatial components of the collected radiation in the Fourier plane, while the components are incident on the filter; and
analyzing the image captured by the image sensor and, responsive thereto, controlling the spatial filter so as to determine the spatial components to be received by the detector.
There is furthermore provided, in accordance with a preferred embodiment of the present invention, a method for spatial filtering, including:
collecting radiation emitted from a point, and separating the collected radiation into spatial components in a Fourier plane;
positioning a programmable spatial filter at the Fourier plane;
capturing an image of the spatial components of the collected radiation in the Fourier plane, while the components are incident on the filter; and
analyzing the captured image and, responsive thereto, controlling the spatial filter so as to block one or more of the spatial components.
There is also provided, in accordance with a preferred embodiment of the present invention, a method for optical inspection, including:
irradiating a spot on a sample with coherent radiation, and scanning the spot along a line on the sample in a scan direction;
collecting a portion of the radiation that is scattered from the spot, and forming a spatial Fourier transform of the collected radiation in a Fourier plane;
positioning at the Fourier plane a programmable spatial filter including an array of mirrors that are controllable so that each mirror can be individually tilted about an axis parallel to the scan direction between a first orientation and a second orientation; and
positioning a detector so as to receive the collected radiation reflected from the mirrors in the array that are in the first orientation, and not the radiation reflected from the mirrors that are in the second orientation, so as to generate, responsive to the radiation, a signal for processing to determine a characteristic of the sample.
There is additionally provided, in accordance with a preferred embodiment of the present invention, a method for spatial filtering, including:
arranging an array of mirrors in a plane; and
controlling the mirrors so that each of the mirrors tilts between a first orientation, in which the mirror is tilted out of the plane so as to deflect radiation that is incident on the mirror toward a detector, and a second orientation, in which the mirror is oriented substantially parallel to the plane so as to deflect the radiation away from the detector.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: